1. Field of the Invention
The present invention relates to method and system for determining antenna at a radio base station at a time of handover in the radio communication such as that of the CDMA scheme that is carried out between a mobile radio terminal and the radio base station.
2. Description of the Related Art
In general, in the communication system in which the radio communication such as that of the CDMA scheme is carried out between a mobile radio terminal and a radio base station, at a time of the handover in which the mobile radio terminal moves from a currently located area to another area, a switching to an antenna of a radio base station that controls a moving target area is carried out and then a synchronization with the radio base station of the moving target area is established.
Conventionally, in this antenna switching related to the handover, when the radio base station of the moving target area has a plurality of antennas, an antenna for transmitting signals to the mobile radio terminal under the handover has been determined fixedly until the synchronization is established.
However, when the antenna for transmitting signals is fixed, there can be cases where a plurality of radio signals to be code division multiplexed are transmitted by an antenna different from that used by the mobile radio terminal that is already located in the moving target area.
When the mobile radio terminal receives such signals that are transmitted from an antenna different from the antenna already in use, there has been a problem that the influence of interferences becomes large and the receiving quality is degraded because the propagation paths are different for different signals. In particular, in the case of despreading the code division multiplexed signals by the RAKE or the like, there has been a possibility of the influence by a phase displacement due to interferences.
As a method for preventing such interferences that can be caused at a time of the handover, there has been a method in which the phase and the transmission power at a time of the transmission are determined such that the signal power received by the mobile radio terminal of the other user becomes small, by using the adaptive array antenna technique.
However, when this adaptive array antenna technique is used, there have been problems that many antennas become necessary in order to suppress interferences sufficiently small, and a control processing and a driving device configuration become complicated.